The present invention relates to wireless communication systems. More particularly, the present invention relates to a method for controlling the forward link power independent of reverse link power control.
In a code division multiple access (CDMA) cellular system, a plurality of mobile stations use the same frequency simultaneously. As such, the signal power of a mobile station has a significant influence on the signal power of other mobile stations.
FIG. 1 illustrates the receive sensitivity of a mobile station as a function of distance between a base transceiver station (BTS) and the mobile station when the BTS transmits at a constant signal strength. In FIG. 1, xe2x80x9cAxe2x80x9d represents a mean path loss, which is determined by the geographical model as well as the distance between the BTS and the mobile station.
It is well recognized by those skilled in the art that the path loss is proportional to the square of distance between the BTS and the mobile station and the variance of the path loss is 8 dB. Also, when a cell radius of 9 Km is assumed, the dynamic range of the path loss is on the order of 80 dB. Further, the mean path loss is constant regardless of link direction and typically has a log-normal distribution.
In FIG. 1, xe2x80x9cBxe2x80x9d represents the strength of the receive signal, which varies according to a Rayleigh Fading distribution, the variation occurs on both the forward link and reverse link, independent of the path loss.
Because the Rayleigh Fading distribution is independent of path loss, some other means of compensation is required for the variation in received signal strength (i.e., B) than what is used to offset path loss. In light of the above, the object of power control is to make the mean power level received in a BTS equal to xe2x80x9cCxe2x80x9d where xe2x80x9cCxe2x80x9d is a standard mean power level required for power control.
To meet the standard mean power level, the power control method employed to combat path loss can be divided into an open loop power control method, a closed loop power control method and an external loop power control method. The external loop power control method is required to perform power control in combination with those power control methods so that the mean receive power level is equal to xe2x80x9cCxe2x80x9d.
If the forward or reverse link status is bad (i.e., the power level has fallen to an unacceptable level), a part of the frame being transmitted through each of the links is susceptible to being erased. In the case where an erasure frame, i.e., a bad frame, occurs on the forward link, the mobile station informs the BTS that the erasure frame has occurred indicating that the forward link is bad. The mobile station then sets the erasure indicator bit (EIB) to xe2x80x9c1xe2x80x9d and transmits the bit to the BTS. This is the process by which the MS informs the BTS that the forward link is bad.
FIG. 2 illustrates the situation where the EIB is set when an erasure frame occurs. As illustrated in FIG. 2, the mobile station 200 and BTS 250 exchange frames of 20 ms duration. The EIB is set and transmitted at an (i+1)th time when an erasure frame occurs in the mobile station 200 at an ith time. Considering a frame delay of 40 to 60 ms which typically occur in a radio wave environment between the mobile station 200 and BTS 250, the EIB arrives at the BTS 250 at the (i+3) time 230 so that the BTS will start power control after the (i+5)th time. Power control does not start until the (i+5)th time in consideration of the time required to analyze the received erasure indicator bit.
Given that the mobile station 200 has transmitted the EIB at the (i+1)th time if the receive sensitivity (i.e., link) of the mobile station should become good at the (i+4)th time, the transmit power level of the BTS cannot be corrected (i.e., increased) until the (i+5)th time due to frame delay. As such, the link remains unadjusted in the intervening time intervals.
That is, a problem occurs in that it takes as many as 5 to 6 frames from the time at which the BTS received the EIB until the BTS increases the power level.
Further, since it is not possible for the mobile station to normally receive a reverse power control bit during the 5 to 6 frames, the quality of the reverse link cannot be guaranteed.
In the case of the conventional method for controlling transmit power, which adjusts the transmit power level of the BTS in accordance with a received message, such as an EIB as described above, the adjusted transmit output of the BTS is not processed in real-time on the basis of a receive frame of the mobile station. That is, there is some lag time between requests of the mobile station and the processing of those requests by the BTS.
In the case where the receive sensitivity of the mobile station becomes good in the intervening time interval between the request and the subsequent power adjustment, the resulting power adjustment is unnecessary and wasteful. Additionally, in the case where the receive sensitivity of the mobile station becomes good for a longer duration, it will inevitably become bad again due to the lag effect. That is, there is a problem in that the BTS can not know the quality of forward link during the time from that the BTS starts to decrease the transmit power to that the mobile station reports the quality of forward link, so the BTS continues to decrease the transmit power even though the quality of forward link becomes worse during the intervening time.
Another conventional method for determining the forward link status, is by using a PMRM (Power Measurement report Message). The mobile station sends a PMRM message to the BTS whenever the quality of the forward link becomes worse. The PMRM includes the number of bad frames received via the forward link.
According, to this method, the mobile station measures the forward frame error rate (FFER) from the frames received via the forward traffic link between the mobile station and the synchronized BTS. If the FFER exceeds a predetermined threshold, the mobile station reports the number of bad frames received via the forward link to the BTS by way of the PMRM.
The PMRM also includes the FFER of the serving BTS measured by the mobile station. Therefore, the BTS can know the transmit power level received by the mobile station as determined from the strength of the pilot signal and frame error rate included in the PMRM.
However in a situation similar to that described above, when the reverse link status is not good, there is no way to control the transmit power level of a BTS, because the control information (i.e., EIB, PMRM) is not sent to the BTS.
If the status of the forward link is not good, the mobile station can not know that an erasure frame has occurred in the reverse link. So, the mobile station can not receive a power control bit, even though the BTS sends the power control bit to direct the mobile station to increase the transmit power as a consequence of receiving the reverse erasure frame. Therefore, if the status of forward link is not good, the status of the reverse link also becomes bad as a result of not being able to receive the reverse power control bit.
That is, if the reverse link is good and forward link is not good, the mobile station can not receive the reverse power control bit from the BTS. Then, the mobile station can not perform reverse power control, accordingly, the reverse link becomes bad. If some bad frames that are more than a predetermined number are received in the BTS as the reverse link is not good, the BTS releases the call.
On the other hand, if the forward reverse link is good and reverse link is not good, the BTS can not receive the information of forward power, i.e., EIB and PMRM. Then, the BTS can not perform the forward power control, consequently, the forward link becomes not good. If some bad frames that are more than a predetermined number are received in the mobile station as the forward link is not good, the mobile station releases the call.
The present invention is a novel method for controlling the forward link power independent of the reverse link power control. The present invention makes it possible for the BTS to control forward link power even though information about the forward link status is not being received by the BTS because the reverse link status is degraded. The method further prevents the BTS from transmitting at an excessive power level in such a situation. Additionally, power control of the forward link is performed independently of the reverse link status, consequently it prevents the reverse link status from becoming more degraded.
According to one embodiment of the present invention, in the case where the reverse link status is not good and, as a consequence, the forward link status is not sent to the BTS, a preferred embodiment of the present invention of the method for controlling forward link power independent of reverse link power control includes the steps of: the BTS making an estimate of the forward link status using power control information, wherein the power control information is received via the reverse link prior to the reverse link status going bad: determining optimal power control information in accordance with a predefined threshold value by repeatedly estimating (i.e., making multiple measurements) the forward link status; improving the forward link status by determining a transmit power of the BTS as a function of the determined optimal power control information; and improving the reverse link status via the improved forward link.
According to another embodiment of the present invention, in the case where both the forward link and reverse link become bad simultaneously, a method for controlling forward link power independent of reverse link power control includes the steps of: improving the forward link status; sending the power control bit of the reverse link to a mobile station via the improved forward link; gradually improving the reverse link status by the mobile station using the power control bit of the reverse link; sending the forward link information to the BTS via the improved reverse link; and controlling the forward link power using the forward link information received via the improved reverse link.
According to another embodiment of the present invention, a method for controlling forward link power independent of reverse link power control includes the steps of: measuring by a mobile station, the pilot signal strength which is received from a BTS, and repeatedly reporting the measured strength and forward frame error rate through the PMRM message to the BTS; transmitting a message to the mobile station on Layer 2 if acknowledgment for the message is not received from the mobile station during a pre-specified time interval since the BTS has re-transmitted the message via the forward link to the mobile station; confirming by mobile station where an erasure frame occurs on the forward link and transmitting an EIB bit via the reverse link to the BTS if the erasure frame occurs on the forward link: confirming by the BTS whether the erasure frame has occurred on the reverse link after the BTS receives at least one PMRM and the EIB from the mobile station; performing power control of the forward link by the BTS using PMRMs and ACK History if the erasure frame has occurred on the reverse link as a result of the confirming step, wherein the PMRMs are received from the mobile station before the erasure frame has occurred and the ACK History occurs in Layer 2 of the mobile station and the BTS; and performing power control of the forward link by the BTS using the EIB received from the mobile station if the erasure frame has not occurred on the reverse link as a result of the confirming step.